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Abstract

Bulk Metallic Glasses (BMGs), also known as amorphous metals, are of considerable scientific and commercial interest due to their random or chaotic structure. Given their potential use as engineering materials, there is a concomitant need to establish their mechanical properties. However, BMGs are not conveniently available in sufficient volumes (especially experimental and combinatorial compositions), making property determination via conventional tensile or compression testing problematic. Instrumented indentation is ideally suited for this purpose because the testing requires only small sampling volumes and can probe multiaxial deformation characteristics at various length scales. In this technique, conducted generally on a sub-micron regime, the depth of penetration of an indenter, usually a diamond, is measured as a function of the applied load and expressed graphically as load (P) - displacement (h) curves from which a host of mechanical properties can be extracted and studied. In this work, a methodology for using instrumented indentation at nano- and micro- scales to determine the mechanical response of BMGs was developed and implemented. The implementation primarily focused on deformation in the elastic regime but included preliminary results related to the onset of inelastic deformation. The methodology developed included calibration techniques, formulations to extract the machine compliances, verifications using standards and verification for uniqueness of instrument deformation under a spherical indenter. The methodology was different for the two platforms used based on the load-depth response characteristics of the instrument. In the case of the Micro Test platform, the load-depth response of the instrument was linear. In the case of the Nano Test platform, the instrument load-depth response followed a 3/2 power law, representative of Hertzian behavior. The load-depth response of the instrument was determined by subtracting the theoretical response from the corresponding raw load-depth response obtained by elastically indenting a standard steel specimen of known modulus. The true response of the sample was then obtained by subtracting the instrument's response from the corresponding uncorrected load-depth response (raw data). An analytical model to describe the load-train compliance was developed. The methodology was verified using quartz and tungsten standards. Indentation experiments were conducted on Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vitreloy 1), Cu60Hf25Ti15, Cu60Zr30Ti10 and Fe60Co7Zr10Mo5W2B16 bulk metallic glasses using spherical indenters with diameters 2.8 mm and 100 [micro]m. The spherical geometry results in a simpler stress distribution under the indenter (when compared to a sharp geometry) and furthermore by recourse to spherical indenters the onset of plastic deformation was delayed. In the case of the Zr-based BMG, the experiments showed that the elastic response did not depend on the diameter of the indenter used indicative of the absence of residual stresses in the sample. Large scale plastic deformation was observed when the sample was indented using a smaller diameter indenter. Log scale analysis (i.e., examining the results on a log load vs. log depth response to check for deviation from Hertzian behavior) showed a deviation from a 3/2 fit indicating a deviation from elastic behavior. The onset implied a yield strength value of ~ 4 GPa, higher than the value reported in the literature (~ 2 GPa). Hence, it is believed that the first signs of plastic deformation occurred at lower loads than the predicted loads from the log scale analysis procedure and is expected to occur as discrete bursts. Discrete plastic events or "pop-ins" were observed in the load-depth indentation responses under quasistatic loading conditions, which were believed to be associated with shear band activity. An attempt was made to formulate a mathematical model based on three yield criteria (Drucker-Prager, Mohr-Coulomb and von Mises). Based on the von Mises predictions and comparable experiments on a quartz standard, it was established that the pop-ins observed were real and not an instrument artifact. Multiple load cycles following partial unload experiments showed that the pop-ins affected the subsequent indentation response. The moduli and the yield strength values obtained for the Cu-based BMGs were comparable to the values reported in the literature. There was significant scatter in the indentation data from the Fe-based BMG. Porosity and lack of 100 % compaction were believed to be the reasons for scatter in the data. The financial support of NSF through grant DMR 0314212 is gratefully acknowledged.

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